Spirometers are used to measure forced vital lung capacity usually of a human subject. A spirometer now in general use includes an expansible chamber having a movable piston fitted with a molded rolling rubber seal which comprises a movable boundary of the expansible chamber. The subject expires air through a tube connected to an opening into the chamber thereby causing the piston to move, expanding the chamber. A recording surface in the form of a removable card is attached to the piston and a pen is drawn across the face of the card at a constant velocity as the subject exhales into the expansible chamber. There is thus obtained on the card a plot of the volume of expiration flow as a function of time. The disadvantage of this type of spirometer is that its readout is purely in mechanical format and hence not readily adaptable to interfacing with computing devices which can perform certain pertinent lung function calculations, given the spirometer measurements.
It is important in using spirometers that any device used to measure the movement of the expansible chamber piston exert an absolute minimum influence thereon so as to prevent distortion of the measurement. For the same reason any means used to convert spirometer mechanical measurements to an electrical format should exert an absolute minimum influence on the quantitative measurements.
According to the present invention a spirometer has a fixed case, a portion of which forms the fixed boundaries of an expansible chamber, and a movable piston fitted with a molded rubber seal which forms the movable boundaries of the expansible chamber so that the piston and fixed case move relative to one another as a function of the quantity of air exhaled by the subject. The piston and case are coupled to one another by means such as a radiant energy transducer affixed to one and indicia on the other which pass through the field of view of the transducer as the piston moves relative to the case and to which the transducer responds to generate a first distinctive cyclically repeating series of digital numbers when movement is in a first direction and a second distinctive cyclically repeating series of digital numbers when movement is in the opposite direction. A logic circuit is provided which combines one of the numbers with the next generated number to form a new number which is analyzed in a look-up table to determine whether a counter should be incremented or decremented. The counter thereby contains a number which is a function of the quantity of air exhaled by the subject.
In the embodiment to be described the radiant energy transducer comprises a pair of reflective object or photo detectors affixed adjacent to one another to the case or to brackets affixed to the case and which view individual fields of view through which indicia in the form of alternate light and dark bars pass as the piston moves. The signal from either one of the reflective object detectors is a lower level sine wave which is conditioned and converted to a rectangular wave. The fields of view are off-set relative to the indicia so that the wave from one detector is out of phase with the wave from the other detector. There is thus instantaneously available from the detectors a two digit word, one digit from each detector, and a distinctive cyclically repeating sequence of four such words for continuous movement of the piston in one direction and the same four words in a cyclically repeating sequence of words for movement in the opposite direction except that the sequence is reversed. As will be explained, two such successively generated words unambiguously denotes that the piston has not moved or has moved less than the indicia pitch, or that the piston has moved one indicia pitch and the direction in which it moved. As explained above, two such successive words are combined to form a new word which is searched for in a look-up table to thereby control the aforementioned counter.
More specifically, the indicia comprise two columns of alternating light and dark bars. The longitudinal axes of the columns are arranged to be parallel to the line of piston movement so that each column passes under a respective reflective object sensor as the chamber expands or contracts. The bars of one column are slightly displaced with respect to the bars of the other column or alternately, a single column is used and the reflective object sensors are slightly displaced with respect to one another along the line of piston movement to allow the logic tests to monitor the direction of piston travel. The alternating light and dark bars of the columns passing under the reflective object sensors cause the outputs of these sensors to switch between the high logic state, (1), and their low logic state, (0). These transitions are counted in appropriate electronic circuits comprised of the aforementioned look-up table and counter to provide a measure of total piston displacement and hence a present volume or differential volume of the expansible chamber in the form of an electrical signal.
It is the main object of this invention to provide an accurate spirometer having an electrical output.
One advantage of the invention is that it can easily be retrofitted to existing spirometers.
Another advantage of the invention is that it allows a spirometer to generate an electrical output signal which can be directly coupled into computing circuits.
A further advantage of the invention is that it provides a means for use in a spirometer for generating an electrical spirometer output signal without mechanically loading the moving elements of the spirometer.